The present invention relates generally to the field of curtain coating and, more particularly, to methods and apparatus for establishing a uniform wetting line on the back surface of a curtain coating hopper lip.
The technique of curtain coating is widely used in the industry of manufacturing photographic films and papers. Typically, a curtain coating apparatus comprises a feed system in the form of one or more slots fed with photographic emulsions and from which the photographic emulsions flow in the form of one or more layers which are superimposed on a slightly inclined plane. The photographic layers then flow onto a lip, where they leave the coating device to form a liquid curtain in substantially vertical free fall. The free-falling curtain is deposited on a moving support web typically while the web is supported on a driven roller. Structurally, the lip is substantially vertical and has a front face on which the layers of photographic emulsion flow, and a rear face forming, with respect to the front face, an angle which is typically around 30xc2x0 to 45xc2x0. The bottom edge of the front face and the bottom edge of the rear face are separated by a bevel, the width of which varies overall between 0.1 mm and 2.5 mm. For applications of this type, the flow rates (per unit width of the lip) vary from 0.6 cm2/s to 6 cm2/s. The viscosity of the photographic layers varies from 0.005 to 3 poise. All these quantities are, of course, mentioned only by way of reference.
Such curtain coating systems have been the subject of numerous patents. By way of example, reference can be made to the European Patent No. EP-A-107 818; U.S. Pat. No. 4,510,882, U.S. Pat. No. 3,632,374; U.S. Pat. No. 3,867,901; and French Patent No. FR-A-2 346 057.
One condition that a curtain coating system can be particularly sensitive to (notably for photographic applications for which uniformity of coating is essential) is the formation of a curtain that is not uniform and homogeneous. This is because a non-uniform curtain creates streaks on the photographic product. That is, the coating is applied to the support web with variations in thickness across the width of the support web. These variations have an appreciable effect on the photographic properties of the film and consequently it is important to minimize such variations.
U.S. Pat. No. 5,725,666 to Baumlin, entitled xe2x80x9cMethod and Apparatus for Improving the Uniformity of a Liquid Curtain in a Curtain Coating System,xe2x80x9d teaches a tool for creating a uniform wetting line on the rear face of the lip of a curtain coater. A perspective view of the tool is shown in FIG. 1. The device comprises two fingers 1, 2 mounted on a frame 3. Each of the fingers 1, 2 defines a first surface 4, 5 designed to be brought to bear on the front face of the lip of the coating device, and a second or rear surface 6, 7 designed to be applied substantially to the rear face of the lip. The first surface forms, with respect to the second surface, an angle substantially equal to the angle formed by the front and rear faces of the lip. Generally, the angle between the two surfaces varies from 30xc2x0 to 45xc2x0. The height of the rear surface 6, 7 of each of the fingers is at least equal to the height over which it is intended that the liquid should wet the rear face of the lip.
During operation, an operator applies the wetting device to the lip of the coating device and slides it so as to cause it to travel at least once over substantially the whole width of the lip. Thus, the rear surface 6, 7 of each of the fingers is applied opposite the rear face of the lip and forces the liquid to wet the rear face of the lip over a height greater than its natural wetting height. There are some problems associated with the use of the device taught by Baumlin. Operator intervention is required. Operation of the tool is manually intensive. Operation of the tool results in generating substantial liquid waste at startup.
U.S. Pat. No. 5,759,633 to Baumlin et al. and entitled xe2x80x9cMethod for Improving the Uniformity of a Liquid Curtain in a Curtain Coating System,xe2x80x9d teaches a method for improving curtain uniformity by forming a liquid curtain over the front face of a lip, progressively reducing the flow rate over the lip to a set value for a period of time so that the rear face of the lip is wet to a greater height, and increasing the flow rate to defined coating conditions. According to the teachings of Baumlin et al., there is initially a liquid composition with a high flow rate (6 cm2/s) and a low viscosity (6.5xc3x9710xe2x88x923P, which typically corresponds to water at 40xc2x0 C. to which surfactants are added to facilitate the formation of the curtain). The flow rate is reduced (1.5 to 2 cm2/s) so as to attain the flow rate level of a wettability window defining a flow rate and viscosity region within which the liquid composition wets the rear face of the lip over a height greater than the natural wetting height over which the coating composition would wet under the operating coating conditions (50xc3x9710xe2x88x922P at a flow rate of 4 cm2/s). There is a progressive change from water to the photographic composition, while the flow rate is held substantially at the reduced value. The change from water to the photographic composition results in an increase in viscosity, which takes place progressively so that the process stays within the wettability window for a sufficiently long period (generally longer than 1second). The viscosity of the coating composition continues to increase outside the conditions of the wettability window. The flow rate is then increased to attain the coating rate. The wetting of the rear face of the lip remains uniform and has an average height of around 0.1 mm.
The location and size of the wettability window are, to a large extent, dependent on the geometry of the lip. Baumlin et al. teaches that for each type of lip there is a corresponding wettability window.
There are some drawbacks associated with the method taught by Baumlin et al. First, the method requires that water precede the introduction of product solutions on the slide surface of the curtain coating apparatus. Further, it is difficult to control the flow rates of the various coating layers in conjunction with the viscosity. The method relies on establishment of the wetting line to substantially wet the back of the lip uniformly across the entire width of the lip. There is also the dependence of the wettability windows on lip geometry requiring that a wettability window be established for each coating lip of different geometry.
Baumlin et al. also teaches a second embodiment of the method. According to this embodiment, a solution of gelatin and surfactant having a viscosity of 0.03P is used. Initially, the curtain is established with a high flow rate (around 6 cm2/s). The rate is then reduced to about 1.5 cm2/s, producing a significant wetting of the rear face of the lip. These conditions are maintained for a few seconds, and the flow rate is again increased to 6 cm2/s. This embodiment of Baumlin et al. has drawbacks similar to those discussed above.
It is therefore an object of the present invention to provide a method and apparatus for generating a uniform wetting line on the rear face of a curtain coating hopper lip.
It is a further object of the present invention to provide a method and apparatus for generating a uniform wetting line on the rear face of a curtain coating hopper lip which is not dependent on wettability windows and hopper lip geometry.
Yet another object of the present invention is to provide method and apparatus for generating a uniform wetting line on the rear face of a curtain coating hopper lip which can be automatically actuated and minimizes waste.
Still another object of the present invention is to provide a method and apparatus for generating a uniform wetting line on the rear surface of a curtain coating hopper lip which does not require physical contact between the apparatus and the hopper lip.
Briefly stated, the foregoing and numerous other features, objects and advantages will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by using a movable trough positioned in close proximity to the hopper lip. The movable trough can be pivoted or moved linearly into a position such that the hopper lip resides proximate to the movable trough. The curtain coating apparatus is then started and the coating solution leaving the hopper lip is intercepted by the trough. The coating solution flowing over the lip fills and floods the movable trough. The flooding of the trough forces the coating solution to substantially wet (to a height on the back side of the lip significantly higher than that of natural product flow) the back side of the hopper lip. The movable trough is then retracted from its position immediately beneath the hopper lip and intercepting the coating solution exiting the hopper lip to thereby allow the free-falling curtain to form and begin impingement on the moving support web to be coated. As the curtain forms, the wetting line on the back of the hopper lip naturally retracts toward the tip of the hopper lip thereby forming a uniform wetting line and a uniform curtain.